With the recent diffusion of large-capacity recording devices, a desired objective has been to increase that the density of magnetic recording. To meet this demand, magnetic metal powders high in coercive force and saturation magnetization have been mainly used.
Further, the recent rapid spread of digital recording media requires further increases in density and improvements in characteristics of the magnetic metal powders, and magnetic recording media complying with these demands have been proposed.
For example, Japanese Patent Unexamined Publication No. 8-102037 discloses "a magnetic recording medium comprising a support and a plurality of layers formed thereon, said layers containing a non-magnetic layer and a magnetic layer, wherein a ferromagnetic powder (A) contained in the magnetic layer contains 2 to 20 parts by weight of Al, 10 to 60 parts by weight of Co and 1 to 16 parts by weight of at least one rare earth element selected from the group consisting of Sm, Y, Pr, Nd and La, based on 100 parts by weight of Fe; the saturation magnetization (.sigma.s) thereof is 130 to 170 emu/g; when the average length of the major axis of the ferromagnetic powder (A) is taken as La (nm), the axis ratio thereof as Xa, the average length of the major axis of the ferromagnetic powder (B) as Lb (nm), and the axis ratio thereof as Xb, 30.ltoreq.La.ltoreq.110, 3.ltoreq.Xa.ltoreq.8, La.ltoreq.Lb, 80.ltoreq.Lb.ltoreq.280 and 5.ltoreq.Xb.ltoreq.12; and the average surface roughness (Ra) of the uppermost magnetic layer is 0.0005 to 0.006 .mu.m". Japanese Patent Examined Publication No. 1-18961 and Japanese Patent Unexamined Publication No. 60-36603 disclose "a magnetic metal powder having a major axis diameter of 0.05 to 0.2 .mu.m and an axis ratio of 4 to 8, wherein the specific surface area measured by allowing N.sub.2 gas to be adsorbed by the BET method is 30 to 55 m.sup.2 /g, and the coercive force Hc is 1300 Oe or more and the saturation magnetization is 120 emu/g or more, for the magnetic characteristics thereof".
In the above-mentioned prior-art magnetic recording medium described in Japanese Patent Unexamined Publication No. 8-102037, the magnetic metal powder has the following composition:
Co: 1 to 60% by weight PA1 Rare earth element: 1 to 16% by weight PA1 .sigma.s: 130 to 170 emu/g PA1 Length of major axis: 0.03 to 0.11 .mu.m
The C/N in the high frequency region is improved by increasing Co. However, an Fe/Co alloy theoretically gives the maximum magnetic energy at 70/30, so that 40% or more of Co is not necessary if Co is homogeneously alloyed with Fe. However, in a magnetic metal powder generally produced, the distribution of Co is non-uniform. Accordingly, 40% or more of Co may be sometimes required for improvements in characteristics, which causes a partial excess of Co in some portions. As a result, the addition of 40% or more of Co is liable to make the characteristics of the magnetic metal powder non-uniform or unstable, resulting in failure to industrially obtain a medium with excellent characteristics.
The magnetic metal powder used herein has a specific surface area by the BET method (hereinafter also briefly referred to as a BET value) as large as 60 m.sup.2 /g and non-uniform as described above, resulting in considerable deterioration of dispersibility. The coated surface is therefore degraded, and the characteristics as a tape are not improved in many cases.
In a magnetic recording medium described in Japanese Patent Examined Publication No. 1-18961, the magnetic metal powder having high magnetic energy is produced by decreasing pores and homogenizing net Fe by specifying the form of spindle-shaped goethite. Because of decreased pores, the BET value is lowered, and the dispersibility is improved, thereby causing little trouble in the production of the medium.
However, the magnetic metal powder in which only the Fe element is used has a limitation on magnetic energy, and is inferior to a magnetic metal powder to which Co is added (Co-addition type magnetic metal powder) in its medium characteristics.
In almost all of the Co-addition type magnetic metal powders, including the magnetic recording medium described in Japanese Patent Unexamined Publication No. 8-102037, spindle-shaped goethite is used as a raw material. The measure for the magnetic recording medium described in Japanese Patent Examined Publication No. 1-18961 does not lead to improvements in the characteristics of the Co-addition type magnetic metal powders, particularly an increase in magnetic energy and a decrease in BET value.